machine/atmega328pb: refactor to actually support this chip

[ysoldak]

This fixes support for atmega328pb chip, code for which does not actually compile now.

The reason is pb variant has two of I2C and SPI interfaces compared with other atmega chips that only have one of each. This results in respective registers naming mismatch and compilation failure.

Also, PWM support for pb variant was implemented only rudimentary, this PR fixes that too.

---

Adding atmega328pb.json target file (see in this PR) and running a simple compilation results in multiple errors this PR fixes.

$ tinygo build -size short -o test.hex -target=atmega328pb examples/blinkm
# machine
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega328pb.go:46:11: undefined: PWM
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega328pb.go:58:11: undefined: PWM
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega328pb.go:97:8: undefined: PB5
...
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega328pb.go:109:8: undefined: PE2
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega.go:40:6: undefined: avr.TWSR
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega.go:40:24: undefined: avr.TWSR_TWPS0
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega.go:40:41: undefined: avr.TWSR_TWPS1
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega.go:46:6: undefined: avr.TWBR
...
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega.go:176:10: undefined: CPUFrequency
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega.go:263:22: undefined: CPUFrequency
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega.go:267:22: undefined: avr.SPCR_SPR0
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega.go:267:38: undefined: avr.SPCR_SPR1
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega.go:269:22: undefined: avr.SPCR_SPR1
...
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega328pb.go:78:7: undefined: PB2
/Users/yursol/Private/code/tinygo/src/machine/machine_atmega328pb.go:83:7: undefined: PB3

[deadprogram]

Passed the HCI tests, so merging. Thanks @ysoldak